1. Field of the Invention
The present invention relates to a long-wavelength laser diode, and more particularly, to a GaInNAs laser diode having an asymmetric waveguide structure.
2. Description of the Related Art
U.S. Pat. No. 5,904,549 discloses a GaInAs laser diode. The GaInNAs laser diode, which is a long-wavelength laser device, has a structure as shown in FIG. 1.
Referring to FIG. 1, the conventional GaInAs laser diode is comprised of a GaAs substrate 6, an n-AlxGa1-xAs lower cladding layer 5, a Ga1-xInxNyAs1-y active layer 4, and a p-AlxGa1-xAs upper cladding layer 3. On the upper cladding layer 3, a contact layer 2 and a p-electrode 1 are formed. An n-electrode 7 is formed on the bottom surface of the substrate 6.
The conventional laser diode has a symmetric waveguide structure in which the upper and lower cladding layers 3 and 5 are formed symmetrically with respect to the intervening active layer 4 having a high refractive index n1. In this case, the upper and lower cladding layers 3 and 5 have a refractive index n2 lower than the refractive index n1 of the active layer 4. Thus, as shown in FIGS. 1 and 2, FIELD is evenly distributed with respect to the p- and n-AlGaAs upper and lower cladding layers 3 and 5, and a considerable portion of the FIELD extends into the upper and lower cladding layer 3 and 5.
If this symmetric waveguide structure is applied to long-wavelength GaInNAs quantum wells on a GaAs substrate, field is densely distributed on a p-cladding layer, so free carrier absorption greatly affects the operation of the device. FIG. 3 is a graph showing free carrier absorption for each wavelength range. As evident from FIG. 3, free carrier absorption increases with an increasing wavelength and in particular absorption significantly increases in the p-cladding layer. Optical loss due to free carrier absorption in the p-cladding layer increases threshold current of a GaInNAs long-wavelength laser diode while reducing slope efficiency that is a change in output with respect to increase in current, thus degrading the performance of the device.